The present invention relates to a technique of decomposing organohalogen compounds by using plasma and, more particularly, to a method of and an apparatus for generating plasma by means of microwaves.
Organohalogen compounds such as CFCs (flon), trichloromethane and halon that include such elements as fluorine, chlorine and bromine in the molecule thereof are used in large quantities in broad applications including refrigerants, solvents and fire extinguishers, and have very high practical values for industrial applications.
However, these compounds have high volatility and may have adverse effects on the environment such as generation of carcinogens and destruction of the ozone layer, when released to the atmosphere, soil, or waters without proper treatment. Thus, the organohalogen compounds must be processed for neutralization in order to protect the environment.
Known methods of the prior art for processing the organohalogen compounds mostly utilize the reaction of decomposition at high temperatures. These treatment processes are roughly divided into burning process and plasma process. In the burning process, organohalogen compounds are burned together with ordinary wastes such as resin.
In the plasma process, the organohalogen compounds are caused to react with steam in plasma thereby to decompose into carbon dioxide, hydrogen chloride and hydrogen fluoride. A method of generating high-frequency plasma by using a high-frequency power source to decompose the organohalogen compounds is proposed in Japanese Patent Application No. Sho 63-284098 (Japanese Patent No. 2134675). A method of generating arc plasma by using a DC power source and applying the plasma for decomposition is proposed in Japanese Patent Application No. Hei 7-3089452 and Japanese Patent Application No. Hei 7-332580.
As to an apparatus that utilizes plasma to decompose organohalogen compounds, an apparatus which utilizes microwave to generate plasma has recently been developed. This decomposition apparatus includes a waste gas processing tank that contains an alkali solution, a reactor tube disposed to open at the lower end thereof in the alkali solution, a circular waveguide that extends vertically above the reactor tube, a discharge tube disposed in the circular waveguide while the lower end thereof communicates with the reactor tube, a rectangular waveguide connected at a position near one end thereof to the circular waveguide and a microwave oscillator mounted on the other end of the rectangular waveguide.
In this decomposition apparatus, CFC gas and steam are supplied to the discharge tube and microwave generated by the microwave oscillator is transmitted through the rectangular waveguide to the circular waveguide. Electric field generated by the microwave inside the circular waveguide causes arc discharge, so that the CFC gas is decomposed by the thermal plasma in the reactor tube. Product gas generated in the decomposition reaction is neutralized while passing through the alkali solution. Other gas containing carbon dioxide is released through an exhaust duct.
Neutralization of the decomposition product gas generates neutralization products such as calcium chloride and calcium fluoride which precipitate in the form of slurry in the alkali solution. The alkali solution is returned to the waste gas processing tank to be reused. In this decomposition apparatus, a solid-liquid separator is installed in the waste gas processing tank where the neutralization product and the alkali solution are separated. The neutralization product separated from the alkaline solution is disposed of as waste.
The decomposition apparatus described above consumes a large quantity of water for cooling the reactor tube and other purposes, as well as the alkali solution. Thus it is desired to reduce the water consumption through improvement of the solid-liquid separation efficiency of the neutralization product and the alkali solution and by decreasing the amount of residual alkali solution included in the neutralization product to be discarded as waste.
While the decomposition apparatus described above has a mixer installed in the waste gas processing tank to mix the alkali solution, the mixer is of the ordinary propeller type that has twisted blades. This mixer can effectively mix the alkali solution, but hardly contributes to the breakdown of bubbles (dividing into fine bubbles) of the decomposition product gas released into the alkali solution, and is therefore not capable of increasing the contact area between the bubbles and the alkali solution. This leads to longer time required for the neutralization reaction and/or larger capacity required of the neutralization tank (waste gas processing tank). It may also result in the generation of acidic gas due to insufficient neutralization.
Moreover, in the decomposition apparatus described above, in case the product gas generated through decomposition of CFC gas is neutralized with the alkali solution over an extended period of time, capability of the alkali solution to neutralize decreases resulting in insufficient neutralization. When the rotation of a motor of the bubble breakdown means decreases the speed thereof or stops altogether for some reason, bubbles in the alkali solution cannot be broken sufficiently, thus resulting in insufficient neutralization. In this case, continuation of the decomposition may cause an acidic gas exhausted out.
The reactor of the apparatus described above includes the reactor tube located above and a blow tube connected to the bottom end of the reactor tube, so that the decomposition product gas (strongly acidic gas) of high temperature (around 1000xc2x0 C., for instance) is blown into the alkali solution through this blow tube. For this reason, a metallic pipe having high heat resistance is normally used for the blow tube. However, when the blow tube is made of a metal, since the metallic pipe is not resistant to acid and alkali despite the high heat resistance, it must be frequently replaced thus leading to higher costs in the equipment and maintenance. When consideration is given only to the resistance to acid and alkali, use of a resin tube may be worthwhile but weak heat resistance thereof has been inhibiting the practical application.
The present invention has been made to solve the problems described above, and a first object thereof is to provide a method of reducing the water consumption in the organohalogen compound decomposition process and effectively and reliably feed CFC gas and other gases.
In order to achieve the object described above, first aspect of the present invention provides a method of decomposing organohalogen compounds by irradiating a gas containing organohalogen compound with microwave to generate thermal plasma, and reacting the organohalogen compounds with the steam in the thermal plasma, said method includes: neutralizing a product gas generated through the decomposition reaction of the organohalogen compounds and steam with an alkali solution, leaving to stand for a predetermined period of time to precipitate a neutralization product generated in this neutralization reaction in the alkali solution, and removing the neutralization product.
With this decomposition process, since leaving the alkali solution, wherein the decomposition product gas has been neutralized, to stand for a predetermined period of time causes the neutralization product to precipitate in the alkali solution, the neutralization product can be easily taken out and the solid-liquid separation efficiency of the neutralization product and the alkali solution can be improved. This makes it possible to improve the yield of recovering the alkali solution from the neutralization product for the purpose of effective reuse, while the quantity of the neutralization product to be disposed of is decreased.
In the decomposition process described above, after stopping the decomposing reaction between the organohalogen compounds and steam, the decomposing reaction may be restarted after removing the neutralization product left for the predetermined period of standing still. In this case, the rate of precipitation of the neutralization product, namely the rate of separation from the alkali solution, can be increased by interrupting the decomposing reaction between the organohalogen compounds and steam thereby suppressing the formation of the neutralization product.
The alkali solution recovered by separating the neutralization product may be reused in the neutralization reaction. In this case, water consumption in the decomposition process can be reduced by reusing the alkali solution separated from the neutralization product that has formerly been discarded.
A suspension may be used for the alkali solution. In this case, use of an alkali suspension containing alkaline substances in a high concentration accelerates the neutralization reaction even when the quantity of alkali solution is small compared to water solution.
The alkali solution may be a suspension containing 20% by weight of Ca(OH)2. This constitution allows the neutralization reaction to proceed efficiently in a shorter period of time.
An apparatus for decomposing organohalogen compounds according to the first aspect of the present invention is an apparatus for decomposing organohalogen compounds by irradiating a gas containing the organohalogen compounds with microwave to generate thermal plasma, and reacting the organohalogen compounds with the steam in the thermal plasma, said apparatus including: a neutralization tank containing the alkali solution that reacts with the product gas generated in the decomposing reaction of the organohalogen compounds and steam, thereby generating the neutralization product; a product gas inlet for introducing said product gas into the neutralization tank and causes the product gas to react with the alkali solution; and a neutralization product removing device that removes the neutralization product which settles in the neutralization tank.
In this decomposition apparatus, when the product gas is introduced into the neutralization tank, the gas reacts with the alkali solution to produce the neutralization product. As the neutralization tank is left to stand for a predetermined period of time in this state, the neutralization product precipitates in the alkali solution whereupon the precipitated neutralization product is taken out of the tank thereby achieving solid-liquid separation.
The neutralization product removing device may have such a constitution that includes a takeout tube of which distal end is put into the neutralization tank while extending to near the bottom thereof, and a suction pump that is connected to the takeout tube and pumps up the neutralization product, wherein the takeout tube has a plate disposed at the distal end thereof in parallel to the bottom surface of the neutralization tank while the takeout tube opens in the lower surface of the plate.
In this decomposition apparatus, the neutralization product precipitated in the tank is pumped up through a clearance formed between the bottom surface of the neutralization tank and the plate. With this configuration, the neutralization product is discharged as it precipitates near the bottom of the neutralization tank, so as to avoid such a situation that the alkali solution is pumped up while leaving the neutralization product at the bottom.
The plate may be a disk having an aperture at the center thereof where the takeout tube opens at the bottom thereof. In this case, since the neutralization product is drawn at substantially uniform speed along the entire periphery of the disk-shaped plate, with the speed being retarded, efficiency of recovering the neutralization product is improved.
A second object of the present invention is to break down the bubbles of acidic gas blown into the waste gas processing tank into fine bubbles, thereby increasing the gas-liquid contact area to accelerate the neutralization reaction.
In order to achieve the object, the second aspect of the present invention is an apparatus for decomposing organohalogen compounds by irradiating a gas containing the organohalogen compounds with microwave to generate thermal plasma, decomposing the organohalogen compounds in the thermal plasma, blowing a product gas generated in the decomposing reaction into the alkali solution in the form of bubbles through a blow tube, thereby to neutralize the product gas, said apparatus including: a bubble breakdown device for breaking down the bubbles in order to accelerate the neutralization reaction.
In this apparatus, since the decomposition product gas that is discharged through the end of the blow tube in the form of bubbles is broken into fine bubbles upon collision with the bubble breakdown device, area of contact with the alkali solution increases and the time taken for the bubbles to reach the liquid surface becomes longer, thereby accelerating the neutralization reaction. Thus such a problem is eliminated as excessive amount of acidic gas is released out of the system due to insufficient neutralization.
The bubble breakdown device may include a shaft that is driven to rotate around a center axis thereof, a blade support hub secured on the shaft at an end thereof, and blades planted on the blade support hub around the periphery thereof substantially at right angles to the plane of rotation. This construction makes the constitution of the bubble breakdown device simpler and reduces the cost, while at the same time allows the bubbles to be broken more finely by the rotating blades or a stream generated by the rotation as the bubbles rise through the alkali solution toward the surface. The rotating blades also mix the alkali solution.
A plurality of the blades may be provided radially on the blade support hub, in which case the effects of breaking down the bubbles and mixing the alkali solution are improved further.
The blade support hub may have a disk shape. In this case, the bubbles rising through the alkali solution first hit the disk of the blade support hub that redirects the bubbles in the radial direction along the bottom surface of the blade support hub aid, when the bubbles depart from the blade support hub, the bubbles hit the rotating blades thereby to be broken into fine bubbles about 3 to 5 mm in diameter.
The shaft, the blade support hub and the blades may all be made of stainless steel, with the blades planted on the blade support hub by silver-alloy brazing. This construction ensures the strength of the bubble breakdown device and reduces the possibility of corrosion thereof by the alkali solution.
The waste gas processing tank that contains the alkali solution may also have baffle plates installed on the inner wall thereof to protrude inward. This construction suppresses the variation of the liquid surface and ensures a stable liquid level thereby to stabilize the neutralization reaction, while at the same time restrains the stream generated in the waste gas processing tank and elongates the detention time of the bubbles thereby ensuring reliable neutralization reaction.
A third object of the present invention is to improve the safety in the process of decomposing the organohalogen compounds such as CFC gas.
A decomposition apparatus according to the third aspect of the present invention has a waste gas processing tank containing a processing solution that neutralizes the decomposition product of the organohalogen compounds and a pH sensor that measures the pH value of the processing liquid.
In the organohalogen compound decomposition apparatus, since the pH sensor measures the pH value of the processing liquid, a decrease in the neutralizing power of the processing liquid can be detected.
An alarm devices may also be provided to issue an alarm when the pH value measured by the pH sensor becomes lower than a predetermined level. Thus personnel can be warned when the pH sensor detects a decrease in the pH value of the processing liquid.
A control device may also be provided to stop the process of decomposing the organohalogen compounds when the pH sensor detects a decrease in the pH value of the processing liquid. The control device stops the process of decomposing the organohalogen compounds when the pH sensor detects a decrease in the pH value of the processing liquid. Thus such a problem is eliminated as excessive amount of acidic gas is released out of the system due to insufficient neutralization.
A decomposition apparatus according to another aspect of the present invention includes a waste gas processing tank containing a processing solution that neutralizes the decomposition product of the organohalogen compounds, a bubble breakdown device that breaks down the bubbles of the decomposition product gas in the processing liquid, and a motor installed on the bubble breakdown device, wherein a motor monitor is provided to monitor the rotation of the motor.
In this organohalogen compound decomposition apparatus, since the motor rotation is monitored by the motor monitor, a decrease in the rotation speed or stopping of the motor can be detected.
An alarm device may also be provided to issue an alarm when the motor monitor detects a decrease in the rotation speed of the motor. Thus personnel can be warned when the motor monitor detects a decrease in the rotation speed or stopping of the motor.
A control device may also be provided to stop the process of decomposing the organohalogen compounds when the rotation speed of the motor measured by the motor monitor is lower than a predetermined value. The control device stops the process of decomposing the organohalogen compounds when the motor monitor detects a decrease in the rotation speed or stopping of the motor. Thus such a problem is eliminated as excessive amount of acidic gas is released out of the system due to insufficient neutralization.
A gas leakage sensor may also be provided to detect gas leakage from the reactor tube. This makes it possible to detect corrosion of the reactor tube as the gas leakage sensor monitors gas leakage from the reactor tube.
An alarm device may also be provided to issue an alarm when the gas leakage sensor detects gas leakage. This makes it possible to issue an alarm to alert the personnel when the reactor tube is corroded and gas leakage from the reactor tube is detected by the gas leakage sensor.
A control device may also be provided to stop the process of decomposing the organohalogen compounds when the gas leakage sensor detects gas leakage. In this case, when the reactor tube is corroded and gas leakage from the reactor tube is detected by the gas leakage sensor, the control device stops the process of decomposing the organohalogen compounds. Thus such a problem is eliminated as release of the acidic gas out of the system is continued.
A booth containing the reactor tube may also be installed, so that the gas leakage sensor detects gas within this booth. In this case, since the reactor tube is enclosed in the booth, the gas that has leaked from the reactor tube is not allowed to escape from the system to ensure safety, and gas leakage can be reliably detected by the gas leakage sensor because the leaking gas does not diffuse.
A draft device may be provided to draw the gas from the inside of the booth, with the gas leakage sensor installed midway in the path of drawing the gas by the draft device. This makes it possible to detect gas leakage more reliably since the draft device collects the gas from the booth while gas leakage sensor installed in the path detects the gas.
A fourth object of the invention is to elongate the service life of the reaction apparatus. In order to achieve this object, the fourth aspect of the present invention is an apparatus for decomposing organohalogen compounds by irradiating a gas containing the organohalogen compounds with microwave to generate thermal plasma, decomposing the organohalogen compounds in the thermal plasma, and blowing a product gas generated in the decomposing reaction into the alkali solution through a blow tube extending from the reactor tube downward, thereby to neutralize the product gas, said apparatus including a cooling liquid discharge device mounted on top end of the blow tube to inject a cooling liquid into the tube.
In this constitution, the cooling liquid, that is injected into the blow tube from the top end thereof by the cooling liquid discharge device, flows down the blow tube thereby quickly cooling down the inside of the blow tube within a tolerable temperature range substantially uniformly. Therefore, the blow tube may be made of a resin such as tetrafluoroethylene. The reason for using the blow tube made of a resin is that the blow tube is required to have resistance against corrosion by the acidic solution that is generated from the acidic gas dissolving in water and resistance against corrosion by the alkali solution contained in the waste gas processing tank, which are difficult to satisfy simultaneously by a metallic tube.
Inside of the reaction tube is always dry and is therefore not likely to be corroded, but is required to have heat resistance. Thus the reaction tube may also be made of stainless steel in order to ensure longer service life.
Instead of providing the blow tube with the cooling liquid discharge device, a detachable joint may be installed between the reaction tube and the blow tube while a cooling liquid discharge device is installed in the detachable joint for injecting the cooling liquid to the inside. In this case, although the detachable joint will be gradually corroded since an acidic solution is generated as the product gas of the decomposition is dissolved into the cooling water, the detachable joint may be replaced with a new one as corroded. Since only the detachable joint is required to be replaced when corroded in the downstream portion of the reactor tube, equipment cost can be reduced further and the replacement operation can be simplified.